I was worried when I was getting a second one too because while Bandit would play with some dogs he was picky about who and sometimes at the dogpark would just "sun" himself lol. When we got Nala he was very uninterested for the first few days, but he did start warming up to her. Now he loves her! He does love that he can get on the couch to escape her puppy hyperness though even though she's over a year now and still hasn't calmed down all that much
My advice would be to make sure you schedule stays the same and his crate and bed are his and off-limits to her (he has a stuffed duck we didn't let her play with too).